Feasibility of high-spatial-resolution nighttime near-IR imaging of Venus’ surface from a platform just below the clouds: A radiative transfer study accounting for the potential of haze

Abstract

We use a customized radiative transfer model to show that sharp ($\sim$10 m resolution) images of the Venus surface can be achieved at night in spectral windows free of CO${}_2$ absorption found between 1.0 and $1.2\mu m$ using a camera at 47 km altitude, just below the planet’s optically thick clouds. This is in spite of the Rayleigh scattering by the dense but still semi-transparent lower atmosphere, and the potential for underlying hazes beneath the clouds. The thermal radiation transmitted directly to the camera forms images of spatially varying surface emissivity and/or temperature at the native sensor resolution, platform stability permitting and under reasonable seeing conditions. Near-isotropic Rayleigh scattering dominates in the $1.0\mu m$ window. Combined with near-Lambertian reflections off the base of the cloud layer, the diffuse light field builds up a background radiance from surface emission averaged spatially out to several 10s of km, i.e., beyond the camera’s field-of-view. At the longer wavelengths (1.1 and $1.18\mu m$ windows), the sub-cloud atmosphere itself partially absorbs (hence less direct light), and therefore weakly emits (hence more background light), but the rapidly decreasing Rayleigh scattering compensates and contrast is maintained. In all cases, we demonstrate that the directly-transmitted surface-leaving radiance from the native sensor resolution element ($\sim$10 m) is a significant fraction of the total radiance, and thus can be detected above the background light. Extending down to the 0.85 and $0.90\mu m$ spectral windows, there is less direct and more background due to the enhanced Rayleigh scattering, but the resulting reduction in contrast can be mitigated by co-adding the $\sim$10 m pixels. This technological advance will open a new era in Venusian geology by enabling discrimination between different surface materials at fine scales. Moreover, potentially active volcanism on our sister planet may be revealed by surface spots that are much hotter than their surroundings.